There is a small probability that a metal halide lamp or a tungsten halogen lamp will burst during operation of the lamp, hereinafter called a "containment failure" of the lamp. When a lamp containment failure occurs, the sequence of events internal to the lamp is as follows: the arc tube or tungsten halogen capsule bursts causing fragments of glass or shards to be propelled against the outer envelope; these shards shatter the outer envelope of the lamp. The external result is that the lamp bursts. It is this type of lamp failure that is the subject of this disclosure.
The causes of these infrequent lamp failures are varied and unpredictable. There is no known way to eliminate the possibility of such failures. Although occurrence of the failure is rare, nevertheless it could present a safety hazard to a person in the immediate vicinity of a lamp. Where such failures can be anticipated, lamp manufacturers notify users by means of warnings on packages and other descriptive materials and by suggested precautions in specifications. This hazard may be avoided by operating the lamp in a fixture designed to contain such a failure. The requirement that the lamp be operated in a protective fixture is frequently employed in commercial usage. However, this procedural safeguard is less acceptable for consumer usage. For reasons of safety, economy, and convenience in both commercial and consumer usage, it may be desirable to incorporate a reliable containment device as part of the lamp itself.
As used herein, the term "light-source capsule" denotes an arc tube of an arc discharge lamp, a halogen capsule of a tungsten halogen lamp, or any light-emitting capsule within the outer envelope of a lamp where the possibility of a lamp-containment failure exists.
The terms "efficacy" or "luminous efficacy" used herein are a measure expressed in lumens per watt of the total luminous flux emitted by a light source over all wavelengths divided by the power input of the source.
In U.S. Pat. No. 4,281,274, issued July 28, 1981, by Bechard et al, there is disclosed an enclosure of glass surrounding the arc tube within the outer envelope of an arc discharge lamp. The enclosure is suggested as being useful as a means to protect against a containment failure of the lamp. While such enclosure may be effective in containing some arc tube bursts, it has been found that in a substantial percentage of cases the enclosure itself is shattered by the burst and containment failure of the lamp follows. Thus, the glass enclosure device taught in the Bechard et al patent offers only limited protection against lamp-containment failures, and such protection is especially tenuous in lamps having light-source capsules in which operating pressures may be as high as 20 or 30 atmospheres.